Radiation therapy is used to treat cancerous tumors with ionizing radiation that kills the affected cancer cells. External beam radiotherapy is one method for delivering the ionizing radiation. In such therapy, a patient is placed on a couch and a radiotherapy beam generator (for example, a linear accelerator) is positioned to direct the ionizing radiation at the patient's tumor. One method for determining the proper positioning of the patient with respect to the beam is to use data from a radiation detector, for example an electronic portal imaging device (EPID). Images from an EPID depict the radiation exiting the patient, essentially providing an x-ray image that can be used to properly locate the patient with respect to the beam. Some modern EPID devices use a phosphor and an array of photosensors to detect radiation exiting the patient. Light from the phosphor is converted to an electrical signal and read by a computer to generate a mapping of the radiation pattern striking the phosphor.